It also helps prevent people from ever redefining them. I mean, if you try to redefine False (in a version of Python that allows it) then you deserve everything that's about to happen to your code... but at the same time, it could possibly lead to a security attack. Redefine False then import some module and get unexpected behavior that you can manipulate to your advantage, somehow. I don't know how that would work, it probably wouldn't... but there's also no reason not to lock those names in and prevent them from ever being redefined.
Could this be so that the interpreter don't inadvertently manipulate them or pass them to a function? param=None and param="" can be very different.